1. Field of the Invention
The present invention relates to a planar monopole antenna and an electronic device.
2. Description of Related Art
In recent years, portable terminals having wireless communication functions, such as handy terminal and PDA (Personal Digital Assistant), have become known to a wide public.
Small-size multi-band antennas capable of sending and receiving wireless signals at a plurality of frequency bands have been known to those skilled in the art. Conventional multi-band antenna is provided with a plurality of antenna elements which resonate at required frequencies to allow a multi-band resonance (for example, see JP-A-2007-13596).
In a large-size monopole antenna, a trap coil is provided at a middle portion of a rod antenna to realize a plurality of resonance frequencies. The trap coil, which is a separate component from the rod antenna, includes a coil and capacitor.
Nowadays, there is a need to use multi-band antennas for wireless communications using portable terminals. However, because the above-described conventional multi-band antenna needs a plurality of antenna elements, a size and an area of the antenna have to be relatively large.
Moreover, assuming that a structure of the above-described conventional monopole antenna provided with a trap coil is applied to a planar film antenna, an additional component separate from the film antenna is required to realize a multi-band antenna. Therefore, it is difficult to attach the component to the film antenna.
Another possible planar antenna may be provided with a plurality of elements in parallel to realize a multi-band antenna. In such a multi-band antenna, however, the distance from a ground plane varies from element to element. Therefore, since impedance depends on frequency, it is difficult to obtain impedance matching properly.